ASoC: rsnd: add CTU support

This patch adds CTU (= Channel Transfer Unit) support on
Renesas R-Car sound driver.
It can Down/Up mixing and splitter. You need to check R-Car datasheet
especially CTUn_CPMDR/CTUn_SV0xR/CTUn_SV1xR/CTUn_SV2xR/CTUn_SV3xR
for setting parameter.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>